1. Field of the Invention
The present invention relates to a compressor device and a control method for controlling a compressor driven by a vehicle engine. The present invention is suitably used for a swash-plate displacement-variable compressor provided in a vehicle refrigerant cycle system.
2. Description of Related Art
For example, EP 0812987 (JP-A-9-324752) describes a compressor device for compressing refrigerant in a refrigerant cycle system. The compressor device includes a swash plate compressor, a pressure control valve and a control unit. The swash plate compressor has a swash plate that is provided in a pressure control chamber and is connected to a piston. The pressure control valve adjusts the refrigerant pressure in the pressure control chamber. The control unit controls an electric current supplied to the pressure control valve in accordance with a thermal load of the refrigerant cycle system, thereby changing an open degree of the pressure control valve. The compressor is driven by motive power from a vehicle engine. Then, the open degree of the pressure control valve is changed in accordance with a current value supplied from the control unit to the pressure control valve, so that the refrigerant pressure in the pressure control chamber is adjusted, and a slant angle of the swash plate, that is, a stroke of the piston is changed. Thus, the displacement of the compressor is changed by controlling the opening degree of the pressure control valve.
However, in the compressor device, the displacement of the compressor is increased or decreased in accordance with the thermal load of the refrigerant cycle system. Therefore, in a case where the compressor is driven by a low-output vehicle engine and the vehicle engine is operated with a low power (low torque) such as in idling, when the displacement of the compressor is changed to a maximum side, the operation torque of the compressor may become larger than the engine torque. In this case, operation of the vehicle engine may be stopped due to the operation torque of the compressor.
Further, in the compressor device, the open degree of the pressure control valve (solenoid valve) is also changed by the refrigerant suction pressure of the compressor. Therefore, it is difficult to accurately control the displacement of the compressor by the pressure control valve.
Further, when a compressor with a large displacement is used in a low-power engine, an engine stall may be caused. Therefore, it is necessary to prepare many different compressor specifications with different displacements, for different power engines.